Accept multiple SQLCipher libraries

[groue]

This pull request aims at easing the job of #1708 and #1816, by checking the SQLITE_HAS_CODEC compiler flag for enabling SQLCipher features.

The GRDBCIPHER compiler flag is now only used for detecting the SQLCipher CocoaPods:

#if GRDBCIPHER // CocoaPods (SQLCipher subspec)
import SQLCipher
#else ...
#endif

#if SQLITE_HAS_CODEC
// GRDB is linked against some flavor of SQLCipher:
// sqlite3_key, PRAGMA cipher_version, etc. are available
#endif

@marcprux, @R4N, I'll merge if you both agree that this pull request is actually useful.

[groue]

Also, @marcprux, I believe this new interpretation of compiler flags removes the need to rework the order of imports, as in b26c42c. Am I correct?

[R4N]

@groue This looks good to us as well. I believe we'll still need to adjust the order so that the SQLCIPHER/SKIP_SQLCIPHER import checks are prior to SWIFT_PACKAGE import check in our PR's since SWIFT_PACKAGE will be true for both:

// Import C SQLite functions
#if SQLCIPHER
import SQLCipher
// #elseif SKIP_SQLCIPHER
// import for SKIP_SQLCIPHER
#elseif SWIFT_PACKAGE
import GRDBSQLite
#elseif GRDBCIPHER
import SQLCipher
#elseif !GRDBCUSTOMSQLITE && !SQLITE_HAS_CODEC
import SQLite3
#endif

we'll sync up our fork once this is merged in and base our PR off of it.

[groue]

The import section was refactored in #1826. This pull request has been rebased on top of it, and does no longer modify the imports.